This invention relates to a focus adjusting device for binocular, and more particularly to a focus adjusting device for binocular in which the center shaft made of cylindrical shaft is formed with a horizontal through hole at a predetermined location of its lower part, and a lever cam made of cylindrical shaft is rotatably inserted into an inner cylinder of the center shaft, and an adjusting knob is rotatably inserted into an outer cylinder of the center shaft, and the lever and the adjusting knob are connected by a set screw through the horizontal through hole of the center shaft so as to move interlockingly, and the lever cam is formed with a screw thread portion in its inner cylinder thereby screwing the thread portion of the lever shaft.
The conventional focus adjusting device of the binocular was constructed in such way that a control lever was shifted vertically by a screw thread mechanism, but the structure forming the focus mechanism of this kind was formed by providing thread in the inner cylinder portion of the center shaft and providing a screw thread portion for screwing with the screw thread portion of the center shaft on a lever shaft and screwing the screw thread portion of the lever shaft with the screw thread portion of the inner cylinder portion of the center shaft. In this kind of the structure, the provision of a screw thread of high pitch in the inner cylinder of the center shaft, for example, a multi-thread screw thread advancing by 30 millimeters in one rotation was technically impossible, and the formation of the screw thread of moderate pitch was regarded inevitable, and as a result, the operation was extremely slow, and in order to shift from an upper limit to a lower limit of the eyepiece lens, it was necessary to turn the adjusting knob many times, and adverse effects accompanied by the rotating operation of the adjusting knob were numerous in various ways which were drawbacks.
As described in the foregoing, as an effort of eliminating the slow operation in the focus adjustment that employs the conventional screw thread mechanism, a focus adjusting device disclosed in U.S. Pat. No. 4,066,329 was proposed. However, this device employed the system of guiding a cam pin formed on a cam shaft by an inclined hole provided on a center shaft so that even if a control lever was rapidly shifted vertically, a fixing and holding force of the control lever was extremely weak which was a drawback. For example, the control lever was naturally dropped as the weight of the eyepiece lens cylinder or the like was applied, or the control lever was shifted by a mere contact of the eyepiece lens slightly on the face of the user during the observation which caused the focus out of order or a phenonemon of dropping the control lever simply when the operation of eye interval by the bodytube of the binocular occurred which put the once obtained focus out of order which, as a result, required frequent operation of adjustment of focus and which were cumbersome matters for the users, and easy occurrence of the displacement of focus resulting from the lack of the fixing and holding force of the control lever was a fatal drawback for the binocular.